Stainless-steel compression tube fittings make it easy to install and maintain measurement and control instruments used in chemical processing, petrochemical plants, paper mills, and many other industrial settings. They can be used to seal a broad range of aggressive fluids and chemicals, and resist internal and external corrosion. The fittings grip and seal by compressing the nose of a ferrule into the outer diameter surface of the tubing. High-quality compression fittings hold internal pressure without leaks or failure until the tubing fails. And users can often repeatedly disassemble and reassemble them with no loss of sealing integrity.
The ferrule may appear rather simple, but actually highly engineered and, to function properly, usually requires considerable design, metallurgy, and production expertise. Not all products on the market meet these stringent requirements. For instance, the ferrule should properly deform elastically and plastically during fitting assembly to properly grip and seal the tubing. Its front edge usually must be harder than the tubing to grip and seal through surface scratches and defects, but if the entire ferrule is too hard, it may not deform properly. Therefore, only the gripping edge of the ferrule may be hardened while the rest may have different, tightly controlled mechanical properties. Also, the hardening process should not compromise stainless steel's corrosion resistance. And finally, production processes should consistently turn out defect-free ferrules that hold tight tolerances and maintain metallurgical specifications.
Ferrules have been machined from cold-drawn stainless-steel bar stock. Cold drawing strain hardens the metal and imparts mechanical strength throughout the ferrule. But the ferrule's front edge was often still not hard enough to seal against tube surface defects such as scratches, weld seams, ovality, and hardness variations, whereas the core hardness was too high to deform properly.
Conventional gas nitriding can be used to case harden the outer surface of the ferrule to a depth of approximately 0.004 in. During assembly, the ferrule front edge shears into the tube. If disassembled, the ferrule remains tightly locked to the tubing, allowing remakes with consistent sealing integrity. However, gas nitriding (as well as carburization and carbonitriding) substantially lowers stainless steel's corrosion
Conventional nitriding and carburizing require high temperatures for the hardening constituents, nitrogen and carbon, to penetrate the passive oxide layer that makes stainless steel corrosion resistant. The high temperatures permit chromium, an anticorrosion alloying element, to diffuse through the metal and form chemically stable nitrides and carbides. These compounds give the surface layer most of its hardness, but in this chemically combined form chromium no longer resists corrosion, and the nitrided or carburized layer corrodes in many environments, including seawater and even moist air.
In addition, nitriding and carburizing can “sensitize” austenitic stainless steel exposed to high temperatures for an extended time. Carbon, which has low solubility in stainless steel, precipitates as chromium carbides in the grain boundaries, depleting regions adjacent to the grain boundaries of the chromium necessary for corrosion resistance. This process is known as sensitization.
Also known are hardening processes that do not reduce the corrosion resistance of stainless steel. These processes create a very hard (generally greater than 60 HRc) but very thin (generally 0.001-0.002 inches or less) ferrule outer skin. The skin allows for consistency in the penetration of a tube's outer surface by the ferrule, which can result in improved sealing and holding. These other processes do not require the high temperatures and long durations that permit chromium diffusion. This keeps chromium in solid solution as a corrosion-resistant alloying element. The hardened layer is continuous, free of defects and voids, as the process tends to fill surface inclusions and substantially reduce end-grain corrosion effects. One such process is the Suparcase® process performed by Parker-Hannifin Corporation.
These other processes also do not affect the bulk metal. There is no sensitization or change in mechanical strength beneath the hardened layer. The ductile layer deforms with the ferrule during assembly without cracking or spalling. In these processes, carbon and/or nitrogen supersaturates the hardened layer. Carbon/nitrogen atoms occupy interstitial sites in austenitic stainless steel's face-centered, cubic crystal lattice, strengthening the hardened layer. The hard crystal-lattice structure would like to expand to accommodate these atoms, but is constrained by the unhardened substrate. As a consequence, high compressive stress further enhances hardness. Compressive stress has the added benefits of substantially increasing a ferrule's fatigue and stress-corrosion resistance. In general terms, the process removes the passive oxide layer from the steel surface, letting carbon/nitrogen atoms diffuse directly into the metal lattice without traversing the passive layer barrier. These atoms diffuse at lower temperatures than other alloying elements, thus avoiding problems caused by formation of carbides and nitrides.
Mechanical properties of the ferrule should also be taken into account. An extremely hard ferrule may be too stiff during assembly and may not bow and properly grip the tubing. But if it is too soft, the underlying material may not support the case-hardened surface. The result can be an eggshell effect: the gripping front edge collapses during assembly and cannot hold the tubing under pressure. It can also reduce a beneficial arcing spring effect.
Cold working can be used to increase hardness and strength of Type 316 austenitic stainless steel after annealing. However, work-hardening rates change with the steel's composition, and constituent percentages can vary within an allowable range. Cold working can also reduce corrosion resistance.